1. Field of the Invention
The invention relates to a method for operating an internal combustion engine in the form of an Otto engine (i.e., a spark ignition engine, carburetor engine), in particular of a motor vehicle. Otto engine fuel—referred to as carburetor fuel or spark ignition engine fuel in the following—in particular gasoline or ethanol (E85), is injected directly into at least one combustion space of the internal combustion engine by means of at least one fuel injector, and the internal combustion engine is selectively operated, instead of with the injection of carburetor fuel or in addition to the injection of carburetor fuel, by means of gas, in particular CNG (compressed natural gas) or LPG (liquefied petroleum gas).
In an Otto engine with direct gasoline injection, the high-pressure injection injectors for injecting liquid carburetor engine fuels issue directly into the combustion space. Consequently, when the engine is in operation, the injectors are acted upon with hot combustion gases as a result of combustion taking place in the combustion space and are heated up. The injectors are cooled by the injected fuel and the engine cooling water ducts running in direct proximity to the injector. When such a direct-injection carburetor engine is operated with CNG (compressed natural gas), as is known, for example, from U.S. Pat. No. 5,755,211 and European patent EP 0 718 484 B1, there is the risk that, during CNG operation, the high-pressure injectors heat up due to the lack of throughput of liquid fuel and are consequently damaged or the fuel still located inside the injectors forms deposits which, in turn, have an adverse effect on the injector behavior. The longer gas operation with deactivated gasoline injection continues, the greater is the risk that deposits from the stationary fuel occur inside the injector and are detrimental to a correct operation of the injector.
It is known from U.S. Pat. No. 6,047,671 and European patent EP 0 761 961 B1, in order to avoid deposits in the injector valve, to add a lubricant and/or cleaning agent to the fuel upstream of the fuel injector or directly in the fuel injector.